1. Technical Field
Illustrative embodiments described in this patent specification generally relate to a drive device that rotates multiple image carriers included in an image forming apparatus, and the image forming apparatus including the drive device.
2. Description of the Related Art
Related-art full-color image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically include multiple image carriers (e.g., a photoconductors) arranged side by side along a direction of movement of a transfer member (e.g., an intermediate transfer belt). Using an electrophotographic method, toner images formed on surfaces of the photoconductors are transferred and superimposed one atop the other on the intermediate transfer belt to form a full-color image according to image data. Thus, for example, chargers charge the surfaces of the photoconductors; an irradiating device emits a light beam onto the charged surfaces of the photoconductors to form electrostatic latent images on the charged surfaces of the photoconductors according to the image data; developing devices develop the electrostatic latent images with a developer (e.g., toner) of respective colors to form toner images on the surfaces of the photoconductors; a transfer device transfers the toner images formed on the surfaces of the photoconductors onto the intermediate transfer belt so that the toner images are superimposed one atop the other to form a full-color toner image on the intermediate transfer belt, and further transfers the full-color toner image onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the full-color toner image to fix the full-color toner image onto the sheet. The sheet bearing the fixed full-color toner image is then discharged from the image forming apparatus.
To meet recent demand for lower costs and space-saving installation, a full-color image forming apparatus in which a single drive motor is used for driving multiple photoconductors has been proposed. Two types of configurations, described below, have been often employed in the full-color image forming apparatus to meet such demand.
FIG. 1 is a schematic view illustrating a first example of a configuration employed in the related-art image forming apparatus. In this configuration, torque from a drive motor is bifurcated by an idler gear to be transmitted to each of multiple photoconductors. Specifically, a motor gear 334 of a drive motor 333 engages a first idler gear 335, and the first idler gear 335 engages each of two second idler gears 336A and 336B so that transmission of torque from the drive motor 333 is bifurcated by the first idler gear 335 into two directions to the second idler gears 336A and 336B. The second idler gear 336A engages each of two drive gears 332A and 332B, and the second idler gear 336B engages a drive gear 332C. Accordingly, the torque is further transmitted to the drive gears 332A; 332B, and 332C, respectively, to rotatively drive respective photoconductors. Thus, the three photoconductors are rotatively driven by the torque from the single drive motor 333.
FIG. 2 is a schematic view illustrating a second example of a configuration employed in the related-art image forming apparatus. In this configuration, torque transmitted from a drive motor to a single photoconductor is sequentially transmitted to the remaining photoconductors via idler gears. Specifically, drive gears 432A, 432B, 432C, and 432D corresponding to four photoconductors engage small-diameter gears 435b, 436b, 437b, and 438b of first idler gears 435, 436, 437, and 438, respectively. The first idler gears 435, 436, 437, and 438 also have large-diameter gears 435a, 436a, 437a, and 438a, respectively, and both the large-diameter gears 435a, 436a, 437a, and 438a and the small-diameter gears 435b, 436b, 437b, and 438b are coaxially provided to the respective first idler gears 435, 436, 437, and 438. The large-diameter gear 435a of the first idler gear 435 is connected to a first photoconductor among the four photoconductors provided at one end of an image forming apparatus in a direction of arrangement of the four photoconductors, and engages a motor gear 434 of a drive motor 433. Second idler gears 476, 477, and 478 are provided between the large-diameter gears 435a, 436a, 437a, and 438a of the first idler gears 435, 436, 437, and 438, respectively, and each of the second idler gears 476, 477, and 478 engages each of two adjacent large-diameter gears 435a, 436a, 437a, and 438a. Thus, torque from the drive motor 433 transmitted to the first idler gear 435 is further transmitted to the remaining first idler gears 436, 437, and 438 via the second idler gears 476, 477, and 478 to rotatively drive the four photoconductors by the torque from the single drive motor 433.
In the first example of the configuration illustrated in FIG. 1, provision of the first idler gear 335 is essential to bifurcate the torque from the drive motor 333. Therefore, compared to the second example of the configuration illustrated in FIG. 2, the first example needs the larger number of idler gears to rotatively drive the same number of photoconductors using the single drive motor 333, thereby increasing the number of components and installation space.
By contrast, in the second example of the configuration, the first idler gears 435, 436, 437, and 438 transmit the torque to the respective photoconductors while transmitting the torque to adjacent photoconductors provided downstream from the corresponding photoconductor in a direction of transmission of the torque. As a result, the number of idler gears can be reduced compared to the first example in which the torque from the drive motor 333 is bifurcated by the first idler gear 335, thereby reducing the number of components, production costs, and installation space.
In general, any eccentricity of a gear along a path to transmit torque from a drive motor to a photoconductor (hereinafter referred to as the torque transmission path or simply transmission path) causes rotary speed fluctuation having a sine curve rotational frequency of that gear in the photoconductor. The rotary speed fluctuation in the photoconductor causes formation of an elongated or contracted latent image on the surface of the photoconductor or transfer of an elongated or contracted toner image onto the intermediate transfer belt from the surface of the photoconductor. Consequently, a full-color toner image formed on the intermediate transfer belt is elongated or contracted. Further, when phase and amplitude of each of the toner images of the respective colors periodically elongated or contracted due to eccentricity of the gears do not coincide with one another on the intermediate transfer belt, color shift occurs upon superimposition of the toner images on the intermediate transfer belt. Even a slight shift in the toner images prominently appears as color shift in a resultant full-color image. Therefore, color shift must be accurately prevented in the full-color image forming apparatus in which the toner images of the respective colors formed on the multiple photoconductors are superimposed one atop the other to form the full-color image.
There is known a method for preventing color shift for the second example of the configuration described above. In the second example illustrated in FIG. 2, the large-diameter gears 435a, 436a, 437a, and 438a of the first idler gears 435, 436, 437, and 438 are provided along the torque transmission path as described above. As a result, eccentricity of the large-diameter gears 435a, 436a, 437a, and 438a causes rotary speed fluctuation having a rotational frequency of the respective large-diameter gears 435a, 436a, 437a, or 438a (or a rotational frequency of the first idler gears 435, 436, 437, or 438) in the respective photoconductors. In order to prevent color shift caused by the above-described rotary speed fluctuation in the photoconductors, rotational positions of each of the first idler gears 435, 436, 437, and 438 are set as follows upon mounting thereof. Specifically, the points of maximum eccentricity in the large-diameter gears 435a, 436a, 437a, and 438a have the same rotational position, respectively, when the same point on the intermediate transfer belt passes each of transfer positions on the photoconductors where the toner images are transferred onto the intermediate transfer belt. Here, rotational position means a rotational angle from the top of the first idler gears 435, 436, 437, and 438 in the vertical direction to a direction opposite a direction of rotation of the first idler gears 435, 436, 437, and 438.
In the above-described mounting method, color shift caused by eccentricity of the large-diameter gears 435a, 436a, 437a, and 438a can be prevented when the large-diameter gears 435a, 436a, 437a, and 438a are individually connected to respective drive motors. However, as described above, the second example has a configuration in which the large-diameter gears 435a, 436a, 437a, and 438a are used not only for transmitting the torque to the respective photoconductors but also for transmitting the torque from the single drive motor 433 to the photoconductors provided on a downstream side in the direction of transmission of the torque. Therefore, color shift caused by eccentricity of the large-diameter gears 435a, 436a, 437a, and 438a cannot be accurately prevented due to the following reasons.
In the second example of the configuration described above, rotary speed fluctuation caused by eccentricity of the large-diameter gears of the first idler gears for all of the photoconductors provided upstream from the remaining photoconductors in the direction of transmission of torque is superimposed on the torque transmitted to the remaining photoconductors provided downstream from the photoconductors in the direction of transmission of torque. Specifically, not only rotary speed fluctuation caused by eccentricity of the large-diameter gear 438a of the first idler gear 438 but also rotary speed fluctuation caused by eccentricity of the large-diameter gears 435a, 436a, and 437a of the first idler gears 435, 436, and 437 is superimposed to cause rotary speed fluctuation in the photoconductor provided at the extreme downstream side in the direction of transmission of torque. Therefore, the phase or amplitude of the rotary speed fluctuation thus generated in that photoconductor provided on the extreme downstream side in the direction of transmission of torque differs from the phase or amplitude of the rotary speed fluctuation in that photoconductor caused only by eccentricity of the corresponding large-diameter gear 438a. In the above-described method, rotary speed fluctuation in the photoconductors caused only by eccentricity of the corresponding large-diameter gear 435a, 436a, 437a, or 438a is considered. Thus, color shift due to rotary speed fluctuation in the photoconductors caused by eccentricity of all of the large-diameter gears of the first idler gears for the photoconductors provided upstream from the remaining photoconductors in the direction of transmission of torque cannot be prevented by the above-described method.
In another approach, the second example of the configuration described above is again employed in an image forming apparatus. In such an image forming apparatus, drive gears coaxially provided to photoconductors are used in place of the idler gears for transmitting torque to the photoconductors, thereby minimizing the number of idler gears and reducing production costs and installation space. However, because the second configuration described above is employed, torque transmitted to the photoconductor provided on the extreme downstream side in the direction of transmission of torque includes rotary speed fluctuation caused by eccentricity of all of the drive gears for the photoconductors provided upstream from that photoconductor in the direction of transmission of torque. Therefore, rotary speed fluctuation due to eccentricity of the drive gears for the photoconductors provided upstream from the other photoconductors in the direction of transmission of torque must be taken into consideration to accurately prevent color shift caused by eccentricity of the drive gears.
In order to accurately prevent color shift, a method for mounting drive gears described below has been proposed.
Specifically, eccentricity proportions of each of drive gears for two adjacent photoconductors is adjusted, and the drive gears are mounted at predetermined rotational positions such that phases and amplitudes of rotary speed fluctuation in the two adjacent photoconductors caused by eccentricity of the drive gears coincide with each other, respectively, when toner images are transferred from each of the two adjacent photoconductors onto the same position on the intermediate transfer belt. Accordingly, not only color shift due to rotary speed fluctuation caused by eccentricity of the drive gears for the corresponding photoconductors but also color shift due to rotary speed fluctuation caused by eccentricity of the drive gears for the photoconductors provided on an upstream side in the direction of transmission of torque can be prevented.
However, in the above-described mounting method, drive gears having a different amount of eccentricity must be manufactured. Further, a combination of the drive gears must be selected such that eccentricity proportions of the drive gears mounted to the two adjacent photoconductors has a predetermined value. Thus, during production of the image forming apparatus, the amount of eccentricity of each of the drive gears must be measured, and the combination of the drive gears that achieves the predetermined eccentricity proportions must be selected, thereby considerably increasing production costs.
In addition, plastic gears formed by, for example, injection molding has come to be widely used as the drive gears and the idler gears in recent years. Eccentricity of the plastic gears is mainly caused by formational error during injection molding. The formational error occurs due to surrounding temperature distribution during formation of the plastic gears, injection temperature distribution of resin, assembly eccentricity of the mold, and so forth. In a recent technology of accurate formation using injection molding, gears in the same lot formed by the same mold substantially have the same amount of eccentricity. Therefore, it is difficult to manufacture the drive gears that can achieve the desired predetermined eccentricity proportions described above.